1. Field of the Invention
The present invention relates generally to the field of composite materials and more specifically to prepreg and laminates made from prepreg that are molded into a wide variety of composite parts. More particularly, the present invention is directed to improving the shelf-life or storability of prepreg, while at the same time reducing the amount and size of voids that form in the composite part during molding operations.
2. Description of Related Art
Composite materials are typically composed of a resin matrix and reinforcing fibers as the two primary constituents. Composite materials are used extensively in the aerospace industry and in other situations where high strength and light weight are desired. Although a wide variety of matrix resins have been used, thermosetting resins, such as epoxy and bismaleimide resins, are particularly popular for aerospace applications. A wide range of fiber types have also been used in aerospace composites. Glass, graphite, carbon and ceramic fiber are common. The fibers can be chopped, randomly oriented, unidirectional in orientation or woven into fabric. The fibers used in composite materials have diameters that range from extremely small to relatively large. Although it is possible to make composites using large diameter fibers, the more common practice is to take thousands of fibers having extremely small diameters and form them into individual bundles known as tows. These multi-fiber tows are much stronger and more flexible than single fibers having the same overall diameter. The tows can be woven into fabric in the same manner as conventional yarns. Alternatively, the tows are arranged in parallel to provide a unidirectional fiber orientation or they can be randomly oriented.
There are a number of ways to combine the resin with the fibers to form the final composite material. One approach, which has been in use for years, is to manually impregnate the fibers with activated resin in-situ on a mold or other support structure. Heat is then used to cure the resulting “lay-up”. This type of manual lay-up procedure is popular because it is simple and requires little, if any special tools. However, it is difficult to accurately control the amount of resin that is applied to the fibers and to insure that the resin is being uniformly impregnated into the fiber tows. In addition, the amounts of curing agent and other additives that are included in the resin may vary between lay-ups. As a result, manual impregnation methods are not typically used in aerospace applications where the combination of high strength and light weight is an important design consideration.
In order to avoid the above problems, it has been common practice to form a prefabricated lay-up (prepreg) that includes the fiber and a resin matrix (resin, curing agents and any additives). The prepreg is made under manufacturing conditions that allow the amount and distribution of resin matrix within the prepreg to be carefully controlled. Once formed, the prepreg may be applied to a mold or other support surface in the same manner as a conventional hand lay-up. In general, prepregs are not used immediately after they are formed. Instead, they usually are stored for use at a later time.
There are a number of characteristics that are desirable in any prepreg. For example, the prepreg must be sufficiently flexible to allow application to the desired mold surface. In addition, the tackiness (or tack) of the prepreg must be such that the prepreg adheres to underlying prepreg layers in the mold while not being so sticky that it becomes disrupted when handled. The prepreg resin should remain stable during storage so that the handling characteristics (i.e. flexibility and tack) do not change. In addition, the resin should not flow away from the fibers or otherwise redistribute itself undesirably during storage. At the same time, the resin should have appropriate flow characteristics during cure to provide good laminate quality.
The stability of prepreg during storage has been, and continues to be, one of the more problematic areas of prepreg manufacture and use. It has been common practice to refrigerate the prepreg during storage and transport to keep reactions between the resin and curative agents to a minimum. However, such refrigeration can be expensive and time consuming. Accordingly, there is a continuing need to provide prepreg systems that are designed to be less dependent on the storage temperature. The goal being to provide prepreg that is designed such that it can be stored and transported at ambient temperatures.
Another area that has been a source of problems for prepreg is the formation of pores in the final laminate. Gas generated during the curing process can become trapped within the laminate where it forms pores or voids that reduce the strength of the final part. There is a continuing need to develop prepreg systems that are designed to vent gas from the prepreg during the curing process to thereby eliminate the formation of undesirable pores.